This invention relates to a balanced vane type oil pump, and more particularly a small, light weight and low cost oil pump wherein a pair of pump cartridges are commonly used as two pumps and the supply of pressurized oil from both pumps is selectively controlled.
An oil pump acting as a source of oil pressure for a power steering device which is used for the purpose of decreasing the handle operating power required for a motor car driver or other engine driven vehicles is usually driven by the engine of the vehicle and its quantity of discharge varies in proportion to the number of revolutions of the engine. For this reason such oil pump is required to have a capacity sufficient to supply a quantity of pressurized oil necessary to operate the power steering device or other load even when the engine rotates at a low speed, in other words, the discharge quantity of the pump is small. However, when the pump is designed to have such a large capacity, where the engine operates at a high speed, the discharge quantity of the pump becomes surplus, which is not only uneconomical but also increases consumption of the engine horse power, thereby increasing fuel consumption.
For this reason, it has been proposed an improved oil pump in which a pair of pump cartridges are used as two pumps of small capacities and a control device having fluid passage transfer function is combined with the cartridges for selectively supplying pressurized oils from the two pumps. More particularly, in this improved arrangement, where the discharge quantity of each pump is small, the sum of the outputs of two pumps is supplied to the load, whereas when the discharge quantity of each pump increases the pressurized oil from only one of the pumps is supplied to the load and to the suction side of the other pump thus circulating the oil. With this measure, it is possible to decrease the horse power required for driving the pumps thus decreasing the horse power consumption.
When constructing such advantageous apparatus, it is necessary to consider a constructional problem which occurs when a pair of pump cartridges consisting of a rotor including vanes and cam rings is used as two pumps.
Thus, since a pair of pump cartridges are used as two pumps, as the simplest construction, may be used a construction wherein a pair of pump chambers formed at positions symmetrical with respect to the rotor axis are separated and connected to separate discharge passages. One example of this construction is disclosed in British Laid Open Patent Specification GB-2038,933A. With this construction, however, although it is possible to simplify the construction of the pump passages and the control device, when one of the pump chambers is connected to the side of an oil tank or reservoir to unload the pump, since only the other pump chamber provides pumping action, an unbalanced load is imposed upon the rotor and its driving shaft so that the durability and the reliability of the movable portions of the pumps are affected. Furthermore such unbalanced load causes noise.
A balanced type oil pump free from this problem is disclosed in U.S. Pat. No. 2,887,060. According to the construction disclosed in this patent, a pair of pump chambers formed about a rotor at positions symmetrical with respect to the axis of the rotor and connected to two independent discharge passages and the outputs of the paired passages opened in respective pump chambers at positions symmetrical with respect to the rotor axis are combined with the pump chambers so as to utilize the pair of pump chambers as independent chambers. This construction, however, increases the number of oil passages with the result that connections of the passages and the pipings to spool valves acting as control devices become complicated.
Since it is usual to incorporate the pump cartridge, the control device and oil passages into a single pump body, various problems described above have a great influence upon the manufacturing, assembling and cost of the entire pump. In addition, this construction increases the overall size of the pump.
In the oil pump of this type, it is desirable to simplify the overall construction and assembling, to make small and light and to decrease the manufacturing cost. Such desire is remarkable especially for a power steering device which is mounted in a small space in an engine room. Consequently, it has been desired to provide an improved oil pump consuming less energy and can fulfill all of these desires.